Attenuation leveling method and apparatus for improved ultrasonic wave propagation

ABSTRACT

Improved ultrasonic wave propagation in connection with operation of ultrasonic imaging systems is made possible by the method of attenuation leveling. Attenuation leveling allows effective focusing of systems that operate through irregular surfaces that cause uneven attenuation effects. Apparatus is provided to implement this method in clinical applications and research applications. The basic attenuation leveling is accomplished with material that attenuates like the body part to be imaged. The apparatus includes conformal surfaces that are in contact with a patient&#39;s body that serve to prevent direct contact of the body with the attenuating material. It also includes fairing surfaces that modify shape of a patient&#39;s body to enable scanning of surfaces. Alternate devices include stand-off devices and immersion configurations.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to creating images by transmitting signals andsensing the effect of objects in the field of view on the signals.

2. Description of the Prior Art

Ultrasonic, or ultrasound, imaging devices depend on signal wavefrontsthat propagate into the body. A wavefront can be described by connectingpoints of equal phase at a given point in time to form a surface that isperpendicular to the direction that ray paths propagate. The wavefrontemanates from a transducer aperture and is shaped by a combination ofthe transducer surface shape and phasing of signals from separateelements of the transducer surface. For focused operation, the wavefrontconverges to a point, except that diffraction prevents perfectconvergence. Successive wavefronts act like new sources of the signalthat are ever smaller and closer to the focus point. The reverse processsimilarly applies to reception where signals from a point source cause awavefront of spherical shape which is ideally received by a sphericalsurface receiver. The body of knowledge of wave propagation physics isrelied on to refine and extend this concept.

Effective system operation requires that signal amplitude along thewavefront be well behaved. Problems arise when attenuation is uneven fordifferent paths such that amplitude variations occur along a wavefront.When a coupling fluid is used to allow waves to travel between atransducer and a body it often happens that uneven attenuationsituations are set up. As the wavefront propagates through a medium,where the medium is a combination of coupling fluid and body tissue,different paths can undergo different attenuation processes. Undesirabledeviations in amplitude can result. This can change the apparentsidelobe levels. Apparent sidelobes are the actual beam responsefunctions that are caused by the propagation effects in combination withideal aperture effects. Apparent sidelobes will be simply calledsidelobes here. This problem is especially significant for highresolution imaging which depends on large apertures and high frequencysignals.

Attenuation means the reduction in signal amplitude other than thereduction that comes from geometric spreading of waves. It is alsoconsidered separately from the opposite effect of signal level increasethat coming from geometric focusing of waves.

Conventional ultrasound practice tends to involve direct contact of atransducer with the skin. Here the body tissue attenuates signals withuniformity over the ray paths to the degree that body tissue is uniform.

It is known in radio frequency antenna design to control power intensityover the antenna aperture with absorbing materials to achieve thedesired radiation pattern. It is also known to taper or weight anaperture, where an aperture is a radiating surface of an array oftransducers, to reduce sidelobe response.

It is known in ultrasound clinical practice to couple ultrasound signalsfrom transducers to the body by use of a water stand-off. Hitachi PartNumber EZU-WL1 is a water bag attachment where the water volume can beadjusted by a syringe. There is a danger with such water filledaccessories because the signals at a focus point are not attenuated forshallow operation as they are naturally for deep operation. Although thepower levels can be adjusted, it is an action that could be easilyforgotten.

Another accessory for oblique stand-off is Hitachi part number EUP-L53STwhich is also water filled.

The disclosure of U.S. Pat. No. 5,902,748 (May 1999) Madsen et al.describes a water bag to couple ultrasonic signals from a transducer toa phantom where the phantom mimicks tissue. This method couples themaximum power to the focal point but may not satisfactorily controlsidelobe response for a variety of phantom shapes.

It is also known to use a tank or bath wherein a fluid couples signalsbetween a transducer and a subject of examination. A coupling fluidmeans that the fluid serves as an ultrasonic transmitting medium. Wateris commonly used as the fluid but a variety of other fluids are used toenable signal coupling. Castor oil is known to match fat for speed ofpropagation, thus preventing refraction at a boundary. Johnson andJohnson baby oil is also known as a good match for breast tissue. Likewater stand-offs in clinical practice, this method couples the maximumamount of power intensity to the focal point but it may not necessarilyproduce the desired control of sidelobe response.

It is known to produce tissue mimicking materials for use in formingultrasound phantoms. U.S. Pat. No. 5,902,748 (May 1999) Madsen et al.discloses useful recipes for making materials that attenuate andpropagate as necessary to represent human body parts.

It is known in manufacturing of composite materials to vacuum bag anassembly to remove air bubbles and cause flexible surfaces to mutuallyconform to each other.

Subjects of examination by ultrasound are commonly human or animal.Other uses are known in other fields.

Referenced documents, in entirety, are incorporated herein. Theycontribute to the description of the present invention, but in case ofconflict, the present document takes precedence.

OBJECTS

A general object is to realize maximum resolution benefits of largeaperture, high frequency ultrasonic imaging apparatus by utilizing anattenuation leveling method. This method provides controlled attenuationover propagation paths so that signals are at desired amplitudes overwavefronts. This would be able to accommodate different human tissuetypes. The same applies to animal tissue.

An object is to provide a signal transmission method that is generallyuseful in clinical practice or laboratory experimental procedure.

An object is to provide safety in ultrasound imaging without causingundue reduction in level of transmitted signals from transducers.

An object is to provide flexible surfaces that comfortably conform tobody parts.

An object is to provide shallow viewing near the skin surface.

An object is to establish a fairing surface that enables effectivescanning by transducer motion.

An object is to establish a fairing surface that simplifies control ofsignal amplitudes that are transmitted from various transducer elements.

An object is to establish a fairing surface that shapes body parts to anacceptable degree so as to enable variations between subjects to beaccommodated.

An object is to enable treatment access simultaneously with real timeimaging.

An object is to establish a laboratory method where uniform wavefrontsare maintained by using attenuating material to fill in paths betweenbody parts and transducers.

An object is to provide pre-compensation to balance attenuationvariations over different paths.

An object is to enable sidelobe control using tapered or weightedamplitude distribution functions.

An object is to combine attenuation compensation with lens functions.

An object is to utilize these methods in industrial inspection and otherfields that involve wave front propagation.

Further objects of the present invention will become apparent from aconsideration of the drawings and ensuing description.

SUMMARY OF THE INVENTION

The invented method is called attenuation leveling and the inventedapparatus enables this method. The basic method involves use ofattenuating fluid to maintain a desired amplitude distribution over thesurface of a signal wavefront as the wavefront travels between atransducer and human tissue.

A combination including a fairing surface, a conformal surface, andspecial coupling fluid is devised to convert a human body surface into asurface that is more amenable to high quality ultrasound imaging.

The present invention is a general method that relates to development ofmaterials disclosed in U.S. Pat. No. 5,902,748 (May 1999) Madsen et al.as the invention of eye glasses relates to development of clear glass.The present invention relates to use of water filled stand-off devicesas the development of eye glasses relates to glass windows.

This invention can be described as a drum where one end of the drum hasa thin rubber sheet fastened to one end of the cylindrical shell of thedrum. The other end of the drum has a sheet of Mylar drawn tightly overthe other end. The cylindrical shell of the drum is actually a taperedcylinder, that is, a hollow cone. This forms a container that is filledwith attenuating fluid that attenuates at the same rate per cm per MHz.as does the body part that is to be examined. The rubber sheet conformsto the body part. The Mylar sheet remains planar. A transducer operatingoutside the drum through the Mylar sheet will produce the same signalamplitude at the focus point regardless of its lateral position. Acoupling fluid efficiently transfers signals from the transducer to theMylar sheet surface. This would be in a container formed on the oppositeside of the Mylar sheet. Better control of the wavefront will beachieved by making this coupling fluid an attenuating coupling fluid.This will give a capability to move the transducer axially while stillmaintaining the same power intensity at the focus point and maintainingthe quality of the focus.

Detailed features include a vacuum method to improve adherence andcoupling between human skin surfaces and the rubber sheet and fluidpressure control. Both the Mylar and the rubber sheet are thin such thatthey have no effect on signal propagation.

An immediate system application involves a transducer that ismechanically scanned. This system enables breast imaging where a varietyof breast sizes and shapes can be accommodated. Mechanical scanning isfacilitated by the fairing surface formed by the taut Mylar sheet.

Another variation is a pre-compensating attenuating pad that allows forvariations in the fluid in which the transducer is immersed.Pre-compensation is an uneven way to control attenuation becausewavefronts are allowed to be uneven in amplitude over differentsuccessive positions and this can give rise to scattering effects.

Variations include use of conventional, hand held ultrasound transducerswith fluid filled cushions that are thin walled, rubber pouches that arefilled from a reservoir or with a syringe. In such cases, the fluid isan attenuating fluid such as evaporated milk. Fluids vary to suit theapplicable tissue type. Fluids can be gels or other firm or solidmaterials as desired.

Safety is improved over conventional water stand-off methods since thepower level transmitted can be kept at the maximum level needed for deeppenetration.

Other variations exclude the fairing function. These include a fluidbath wherein both a transducer and a subject of examination are immersedand the fluid is an attenuating fluid that enables uniform amplitudewavefront.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The scope of the invention should be determined by the appended claimsand their legal equivalents and not by the examples and variationsgiven. Actual medical practice would be expected to result in manyvariations of this concept.

DRAWINGS

FIG. 1—an apparatus to compensate for tissue attenuation effects thatprovides for fairing of irregular body surfaces and mechanicaloperation.

FIG. 2—an attenuating safety stand-off with reservoir to maintainattenuation fluid in stand-off pouch.

FIG. 3—an attenuating safety stand-off with capability to be filled bysyringe.

FIG. 4—an experiment set-up with a tissue mimicking phantom in a tank.

The invented method guides the process of leveling the effects ofattenuation on wavefronts. From a radiating surface 6 indicated in FIG.1 to a focus point 18 in the same FIG. 1 a set of many, evenlydistributed paths can be drawn that are called ray paths. The method isto insert materials that may be solid or liquid in way of such ray pathsso as to cause uniform attenuation over all such ray paths. Certaintapering of attenuation is also appropriate. The simplest way to insertsuch material is to allow an attenuating fluid to fill in uneven bodyterrain where the attenuating fluid is matched in attenuation to thebody attenuation.

A combination of a fairing surface, a conformal surface, and specialcoupling fluid is devised to convert a human body surface into a surfacethat is more amenable to high quality ultrasound imaging, with an addedbenefit of safety. In some forms, the fairing surface is establishedwith a stiff barrier surface. In the simpler forms, the fairing surfaceis established by the shape of the transducer face in contact with aflexible membrane. The special fluid couples energy between body andtransducer as well as matches the attenuation of human tissue. This isuseful where the body surface is uneven or an angle not perpendicular tothe skin is desired. It also allows more freedom of movement insearching and optimizing an image, though contact with the stand-offdevice needs to be maintained.

The preferred embodiment is described here in reference to FIG. 1. Askin surface 1 is in close contact with a thin latex rubber sheet 2which is sealed to a tapered container 4. Sides of tapered container 4are tapered to accommodate large aperture illumination. The rubber sheet2 is shown conformal to the skin 1 even though the body part isirregular as might be a female breast. A Mylar sheet 5 is taut againstthe opposite end of the tapered container 4. This tapered container isfilled with attenuating fluid 11 such as evaporated milk. Materialswould be derived from the body of knowledge, that includes recipes ofU.S. Pat. No. 5,625,137 (May 1999) Madsen et al., for making ultrasoundphantoms. A generic transducer 7 is immersed in a coupling fluid 22contained in upper container 21 to a level 15, but the coupling fluidcan alternatively be an attenuating fluid like the lower attenuatingfluid 11. A transducer radiating surface 6 creates a wavefront thatpropagates toward and converges at a point 18 subject to diffractionlimitations. Wavefront convergence is indicated by boundary line 3 alongwith a like boundary on the opposite side. A pad 20 that represents anattenuating material is used for pre-compensation for coupling fluid 22that does not attenuate, though it is not needed for coupling fluid thatattenuates. The same pad 20 alternately, or in combination with anattenuation function, represents a refracting lens function thatoperates like curvature and time delay methods to focus beams. Thegeneric transducer 7 moves laterally as indicated by double headed arrow8 and axially as indicated by double headed arrow 9. A generic mechanism14 to mechanically move the transducer is indicated without detail. Asealing device 10 stops air leakage around the skin perimeter and alsotransfers vacuum around that perimeter. This causes the attenuatingfluid to take the shape of the body part with only the thin rubber sheet2 as a barrier. The rubber sheet 2 is of thickness that is less than onefourth wavelength for ultrasound in that material so it is of negligibleultrasonic effect. The same rule applies to the Mylar sheet 5.Attenuating fluid 11 is allowed to freely transfer via tubing 12 betweenits container 4 and the reservoir 13 where the reservoir is vented atthe top to atmospheric pressure.

The illustrated apparatus accomplishes the primary object of thisinvention because all paths drawn from radiating surface 6 and focuspoint 18 undergo the same attenuation magnitude.

An added benefit is safety since the power intensity at focal point 18is the same for all lateral positions of the transducer, in contrast toconventional coupling liquids that do not attenuate. For a watercoupling fluid, a An added benefit is safety since the power intensityat focal point 18 is the same for all lateral positions of thetransducer, in contrast to conventional coupling liquids that do notattenuate. For a water coupling fluid, very strong signal would occurwhen the transducer 7 was in the position shown. An added benefit issafety since the power intensity at focal point 18 is the same for alllateral positions of the transducer, in contrast to conventionalcoupling liquids that do not attenuate. For a water coupling fluid, verystrong signal would occur when the transducer 7 was in the positionshown. An added benefit is safety since the power intensity at focalpoint 18 is the same for all lateral positions of the transducer, incontrast to conventional coupling liquids that do not attenuate. For awater coupling fluid, very strong signal would occur when the transducer7 was in the position shown. An added benefit is safety since the powerintensity at focal point 18 is the same for all lateral positions of thetransducer, in contrast to conventional coupling liquids that do notattenuate. For a water coupling fluid, very strong signal would occurwhen the transducer 7 was in the position shown very strong signal wouldoccur when the transducer 7 was in the position shown.

Part of the purpose of this apparatus is to fair in the natural bodyshape to create a smooth interface while maintaining quality of focusand apparent sidelobe levels. Comfort to the patient is provided byavoiding direct contact with fluid and avoiding any significantpressures against the body. The vacuum process is benign because therubber sheet 2 is compliant. The fairing effect of the Mylar sheet 5means that a larger breast will be slightly pressed down while a smallerbreast might not even reach the height of the Mylar sheet 5. In eithercase, high resolution imaging would be maintained.

The transducer face 6 is shown with curvature that matches a wavefrontcurvature that will focus at focus point 18. In cases where flat arraysare used and time delay is used to form a wavefront, the attenuation ofan attenuating fluid 22 in the upper container 21 will not be correctunless signals from transducer elements are correctly adjusted. Thisadjustment will vary with different locations of focal point 18 and itis part of the problem of beam steering in general.

The Mylar sheet can be stretched tight over more complicated frames soas to better apply to some body parts. If a frame is rectangular box,open at top and bottom, but two opposite ends of the box are shaped ascurved arches, then the stretched Mylar surface will be curved in onedimension and this surface will be like a section of a hollow cylinder.Such design variations are suitable for operation with some transducerdesigns.

A system design will utilize the methods, forms, and materials describedin FIG. 1 as needed for particular applications.

A simplified variation is a hand held form as illustrated in FIG. 2.Here a bladder device made of thin rubber, as specified for the rubbersheet 2 of FIG. 1. This apparatus retains the same reservoir 13 and tube12 as before. The reservoir elevation would be higher than the bladderdevice 16 to keep most fluid in the bladder 16. The transducer 7 is nowhand held and lateral motions 8 and axial motion 9 are manually carriedout. Operation of this arrangement is made flexible because the amountof fluid in the pouch can vary so that contact with the pouch will bemaintained for a substantial range of axial motion of the transducer. Ifelectronic system settings are kept unchanged, the image frame can besafely moved in the axial direction as needed and moving across unevenbody contours can be safely done.

The hand held method is again illustrated in FIG. 3. Here the reservoiris eliminated and the bladder 16 is filled with attenuating fluidthrough a rubber plug 17 with a syringe. A particular form isimplemented using Hitachi Part Number EZU-WL1 with evaporated milk beingthe fluid that is inserted with the syringe after any water therein isremoved. Use of this device as herein modified will prevent accidentaluse of high intensity ultrasound when using it with a stand off to seeshallow features in the body. It will also assure that all ray pathsfrom the transducer face 6 to the focus point 18 are equally attenuated.

When a conventional ultrasound transducer such as illustrated in FIG. 2or FIG. 3 is operated with its associated electronic system theappropriate phase and amplitude control of signals is provided such thatimaging is effective from near the skin down to a particular depth. Withthe herein disclosed device the same phase and amplitude control wouldbe maintained so the image frame would be displaced to a shallower rangeof depths and part of the image frame would thus be in the stand offfluid. This can result in viewing in a more effective part of a frame,it can result in the image frame accommodating irregular body surfaces,and it can enable viewing at angles not perpendicular to the skin.Because water does not attenuate signals like body tissue attenuatessignals, image signals are not adequately maintained over the apertureand image quality is degraded.

It is appropriate to provide an ultrasound conducting gel to assurecontinuous contact with the skin. This is desirable, even with thevacuum system in operation. Where this is a thin layer, it is notnecessary that this material be an attenuating material. As operatingfrequencies increase, it will be necessary to evaluate the degree ofsurface irregularity and to utilize attenuating gel should attenuationleveling be appropriate.

FIG. 4 illustrates an experimental arrangement that illustrates basicprinciples of the attenuation leveling method as well as an embodimentof the invented apparatus. A glass tank 31 contains a coupling fluid 32and a transducer array 33 includes a plurality of vertical elements. Acircle 34 is a visualization aid to show how the array elements arearranged to focus at a focus point 39 along a vertical line 38. Anabstract body part is depicted by the hemispherical container 37 that isshown as a wire frame model that is enclosed by a thin rubber surfacethat is the shape of the frame. The body part is called a phantom and itis modeled by filling the container 37 with fluid that acts like breasttissue. The edge ray path 35 travels a shorter part of its route throughtissue than does the central ray path 36. By using coupling fluid 32that attenuates like tissue, both path undergo the same attenuation. Anattenuating coupling fluid is formulated utilizing the same recipes thatare disclosed for making tissue mimicking phantoms as given in U.S. Pat.No. 5,902,748 (May 1999) Madsen et al. This coupling fluid assures thatthe wavefront that is initially produced by the array of transducers 33is correctly maintained as it converges to the focus point 39.

The herein disclosed method and apparatus have utility in any fieldwhere attenuation of propagating signals modifies the intensitydistribution of signals over the surface of a wavefront.

What is claimed is:
 1. A sensing system for examining a subject, saidsensing system comprising (a) a transducer device that operates inrelation to propagating wavefronts, and (b) said propagating wavefrontspropagate in a medium that causes signal attenuation that variablyattenuates signals that are distributed over said wavefronts, and (c)attenuation leveling material placed in the path of said propagatingwavefronts that compensates for variable attenuation to control signalamplitude for said signals that are distributed over a wavefront, and(d) a fairing device placed in the path of said propagating wavefrontsthat reduces uneven surfaces of said subject of examination, and (e) aconforming surface placed in the path of said propagating wavefrontsthat prevents direct contact of a surface of said subject of examinationand said attenuation leveling material.
 2. A sensing system according toclaim 1 where said wavefronts represent ultrasonic waves.
 3. A sensingsystem according to claim 1 and a vacuum device adapted to act on saidconforming surface that improves contact between said conforming surfaceand said surface of said subject of examination.
 4. A sensing systemaccording to claim 1, and contact enhancing gel coupled to saidconforming surface that improves transfer of ultrasonic wave signalsbetween said conforming surface and said surface of said subject ofexamination.
 5. A sensing system according to claim 1 where saidattenuation leveling material is an attenuating fluid.
 6. A sensingsystem according to claim 1 where said attenuation leveling material isan attenuating fluid that has low scattering properties.
 7. A sensingsystem according to claim 1 where said attenuation leveling material isan attenuating fluid having properties such that magnitude of scatteringdoes not add to an effect of scattering of a tissue type being examined.8. A sensing system according to claim 1 where said attenuation levelingmaterial is evaporated milk.
 9. A sensing system according to claim 1where said attenuation leveling material is a gel.
 10. A sensing systemaccording to claim 1 where said fairing device is a flat Mylar sheet.11. A sensing system according to claim 1 where said fairing device is athin sheet of material stretched over a frame so that it is shaped bysaid frame.
 12. A sensing system according to claim 1 where saidconforming surface is a thin rubber sheet.
 13. A sensing systemaccording to claim 1 where said system is adapted for focusingultrasonic waves in breast tissue.
 14. A sensing system according toclaim 1, where said transducer device is adapted to operate withinattenuating fluid.
 15. A sensing system according to claim 1, where saidtransducer device is adapted to move in a direction that isperpendicular to a line from said transducer device to said subject ofexamination, and focused wave power intensity is approximatelyunchanged.
 16. A sensing system according to claim 1 where saidtransducer device is adapted to move in a direction that is parallel toa line from said transducer device to said subject of examination, andfocused wave power intensity is approximately unchanged.
 17. A sensingsystem according to claim 1 where said attenuation leveling material hasproperties such that an attenuation effect is achieved that isapproximately the same rate per cm per MHz as an attenuation effect insaid subject of examination.
 18. An ultrasonic system comprising (a) atransducer that operates in relation to waves in a subject material,where said waves are related to said transducer through a medium bypropagation through said medium, where said propagation is described bywavefronts that travel between said transducer and points in saidsubject material, where signals that are distributed over a wavefronthave relative magnitude that conforms to an amplitude distributionfunction, where effectiveness of system operations depends onpropagation where such signal distributions do not deviate from desiredamplitude distribution functions, and (c) a conforming arrangement ofdifferent materials that implements said medium to enable saidpropagation, where said different materials include a coupling materialwhere said coupling material intervenes between said transducer and saidsubject material, and where said conforming arrangement causes pathlength variations in said subject material such that, for a saidcoupling material that does not attenuate, said path length variationswould cause deviation from a desired amplitude distribution function,and (d) said coupling material having an attenuation effect such thatsaid coupling material is an attenuation leveling material, where saidleveling attenuation effect matches an attenuation effect of saidsubject material so as to provide approximately uniform attenuation insaid medium such that significant deviation from said desired amplitudedistribution functions is prevented.
 19. An ultrasonic system accordingto claim 18 where said attenuation leveling material is a couplingfluid, and fluid properties of said coupling fluid allow said transducerto be moved, and said transducer continues to effectively operate inrelation to said waves in a subject material.
 20. An ultrasonic systemaccording to claim 18 where said transducer is adapted to move to changeposition of said waves in a subject material to cause said wavefronts,and said desired amplitude distribution functions apply to said variedwavefronts.
 21. An ultrasonic system according to claim 18 where saidleveling rate of said attenuative leveling material is such that itattenuates at approximately the same rate per cm per MHz as saidattenuation effect in said subject material.
 22. An ultrasonic systemaccording to claim 18 where said desired amplitude distributionfunctions are constant valued functions.
 23. An ultrasonic systemaccording to claim 18 where said waves in a subject material are focusedin said subject material.
 24. An ultrasonic system according to claim 18and a conforming barrier coupled to said attenuation leveling materialto isolate said attenuation leveling material from a surface of saidsubject material.
 25. An ultrasonic system according to claim 18 wheresaid attenuation leveling material is a coupling fluid that conforms tosaid subject material, and a conforming barrier coupled to said couplingfluid to isolate said coupling fluid from a surface of said subjectmaterial, and contact enhancing gel coupled to said conforming barrierthat improves transfer of ultrasonic wave signals between saidconforming barrier and said surface of said subject material.
 26. Anultrasonic system according to claim 18 where said transducer adapted tomove in a direction that is approximately perpendicular to a linebetween said transducer said subject material, and said waves have apower intensity at a focus point that is approximately constant.
 27. Anultrasonic system according to claim 18 where said transducer adapted tomove in a direction that is approximately parallel to a line betweensaid transducer and said subject material, and said waves have a powerintensity that is approximately constant.
 28. An ultrasonic systemaccording to claim 18 where said transducer is adapted to move changeposition of said waves in a subject material to cause said wavefronts,to vary and said desired amplitude distribution functions apply to saidvaried wavefronts where said desired amplitude distribution functionscause a system response that includes a relative sidelobe response ofsaid system, where said relative sidelobe response is the ratio of asidelobe response to a focused response, and said desired amplitudedistribution functions are formulated to cause said relative sideloberesponse to be lower than a relative sidelobe response for amplitudedistribution functions that are uniform.
 29. An ultrasonic systemaccording to claim 18 wherein said subject material comprises a tissuemimicking phantom.
 30. An ultrasonic system according to claim 18wherein said system is adapted to operate as a research apparatus tomeasure a transducer function.
 31. An ultrasonic system according toclaim 18 and a fairing surface placed in the path of said wavefrontswhere said fairing surface modifies said shape of said subject material.32. An ultrasonic system according to claim 18 wherein said conformingarrangement comprises a conforming surface to isolate said attenuationleveling material from a surface of said subject material, and a saidsystem further comprises a vacuum device to cause said conformingsurface to come in close contact with said subject material.
 33. Anultrasonic system according to claim 18 that adapted to focus wavesignals in human subjects.
 34. An ultrasonic system according to claim18 that adapted to focus wave signals in animal subjects.
 35. Anultrasonic system according to claim 18 where said conformingarrangement attenuation leveling material is adapted to be confinedwithin a container such that said subject material may be immersed insaid container.
 36. An ultrasonic stand-off pad that couples wavesignals between a transducer and a body to be examined, and saidstand-off pad being adapted to conform to a surface of said transducerand to conform to a surface of said body to be examined thereby definingconforming surfaces of said pad, where said body to be examinedattenuates said wave signals, and said wave signals are defined byrespective wavefronts, where signals are distributed over a wavefront,where said stand-off pad contains attenuating material havingattenuation that approximately matches attenuation of said body to beexamined, such that significant uneven attenuation of signals that aredistributed over a respective wavefront is prevented.
 37. An ultrasonicstand-off pad according to claim 36 where said attenuating materialattenuates at a rate in dB per cm per MHz, and said body to be examinedattenuates at a rate in dB per cm per MHz, and these rates areapproximately equal.
 38. An ultrasonic stand-off pad according to claim36 where said stand-off pad is adapted to cause signals that aredistributed over wavefronts to be in conformance with an intendedamplitude distribution function that is pre-determined.
 39. Anultrasonic stand-off pad according to claim 36 where said stand-off padis adapted to conform for shallow operation and deep operation ofultrasonic equipment, and attenuation of said pad is selected to enablesafe said shallow operation of ultrasonic equipment at a transmittedpower level that is the same as a transmitted power level needed forsaid deep operation.
 40. An ultrasonic stand-off pad according to claim36 where said pad is a container that contains attenuating material thatis a fluid.
 41. An ultrasonic stand-off pad according to claim 36 wheresaid pad is a container that contains evaporated milk.
 42. An ultrasonicstand-off pad according to claim 36 where said pad is a container thatis equipped with a port that enables filling with a syringe.
 43. Anultrasonic stand-off pad according to claim 36 where said pad is acontainer that is connected to a reservoir by a tube so that attenuatingfluid can allow volume of said pad to vary.
 44. An ultrasonic stand-offpad according to claim 36 and a vacuum device coupled to said pad andadapted to assure close contact of said stand-off pad with a surface towhich it must conform.
 45. An ultrasonic stand-off pad according toclaim 36 where said transducer is adapted to operate at an angle notperpendicular to said surface of said body to be examined, andflexibility of said stand-off pad is selected to enable it to continueto conform as specified.
 46. An ultrasonic stand-off pad according toclaim 36 where said transducer is adapted to operate in relation to anuneven surface of said body to be examined.
 47. An ultrasonic stand-offpad according to claim 36 wherein said standoff pad is adapted to enablemovement of a focus point within said body to be examined.
 48. Anultrasonic stand-off pad according to claim 36 and coupling gel coupledto at least one of said conforming surfaces to improve signal transferbetween said conforming surfaces.
 49. An ultrasonic stand-off padaccording to claim 36 where said pad is a container made of thin,rubber-like material.
 50. An ultrasonic stand-off pad according to claim36 where said pad is an attenuating gel having sufficient viscosity thatit remains between a transducer and skin of a subject and conforms tosurfaces thereof.
 51. An ultrasonic stand-off pad according to claim 36where said pad is adapted to focus quality as a focus point is movedwithin said body to be examined.